Azithromycin Resistance in Shigella spp. in Southeast Asia.

Infection by Shigella spp. is a common cause of dysentery in Southeast Asia. Antimicrobials are thought to be beneficial for treatment; however, antimicrobial resistance in Shigella spp. is becoming widespread. We aimed to assess the frequency and mechanisms associated with decreased susceptibility to azithromycin in Southeast Asian Shigella isolates and use these data to assess appropriate susceptibility breakpoints. Shigella isolates recovered in Vietnam and Laos were screened for susceptibility to azithromycin (15 µg) by disc diffusion and MIC. Phenotypic resistance was confirmed by PCR amplification of macrolide resistance loci. We compared the genetic relationships and plasmid contents of azithromycin-resistant Shigella sonnei isolates using whole-genome sequences. From 475 available Shigella spp. isolated in Vietnam and Laos between 1994 and 2012, 6/181 S. flexneri isolates (3.3%, MIC ≥ 16 g/liter) and 16/294 S. sonnei isolates (5.4%, MIC ≥ 32 g/liter) were phenotypically resistant to azithromycin. PCR amplification confirmed a resistance mechanism in 22/475 (4.6%) isolates (mphA in 19 isolates and ermB in 3 isolates). The susceptibility data demonstrated the acceptability of the S. flexneri (MIC ≥ 16 g/liter, zone diameter ≤ 15 mm) and S. sonnei (MIC ≥ 32 g/liter, zone diameter ≤ 11 mm) breakpoints with a <3% discrepancy. Phylogenetic analysis demonstrated that decreased susceptibility has arisen sporadically in Vietnamese S. sonnei isolates on at least seven occasions between 2000 and 2009 but failed to become established. While the proposed susceptibility breakpoints may allow better recognition of resistant isolates, additional studies are required to assess the impact on the clinical outcome. The potential emergence of azithromycin resistance highlights the need for alternative options for management of Shigella infections in countries where Shigella is endemic.

Clinical implications of reduced susceptibility to fluoroquinolones in paediatric Shigella sonnei and Shigella flexneri infections.

OBJECTIVES: We aimed to quantify the impact of fluoroquinolone resistance on the clinical outcome of paediatric shigellosis patients treated with fluoroquinolones in southern Vietnam. Such information is important to inform therapeutic management for infections caused by this increasingly drug-resistant pathogen, responsible for high morbidity and mortality in young children globally. METHODS: Clinical information and bacterial isolates were derived from a randomized controlled trial comparing gatifloxacin with ciprofloxacin for the treatment of paediatric shigellosis. Time-kill experiments were performed to evaluate the impact of MIC on the in vitro growth of Shigella and Cox regression modelling was used to compare clinical outcome between treatments and Shigella species. RESULTS: Shigella flexneri patients treated with gatifloxacin had significantly worse outcomes than those treated with ciprofloxacin. However, the MICs of fluoroquinolones were not significantly associated with poorer outcome. The presence of S83L and A87T mutations in the gyrA gene significantly increased MICs of fluoroquinolones. Finally, elevated MICs and the presence of the qnrS gene allowed Shigella to replicate efficiently in vitro in high concentrations of ciprofloxacin. CONCLUSIONS: We found that below the CLSI breakpoint, there was no association between MIC and clinical outcome in paediatric shigellosis infections. However, S. flexneri patients had worse clinical outcomes when treated with gatifloxacin in this study regardless of MIC. Additionally, Shigella harbouring the qnrS gene are able to replicate efficiently in high concentrations of ciprofloxacin and we hypothesize that such strains possess a competitive advantage against fluoroquinolone-susceptible strains due to enhanced shedding and transmission.

A multi-center randomized trial to assess the efficacy of gatifloxacin versus ciprofloxacin for the treatment of shigellosis in Vietnamese children.

BACKGROUND: The bacterial genus Shigella is the leading cause of dysentery. There have been significant increases in the proportion of Shigella isolated that demonstrate resistance to nalidixic acid. While nalidixic acid is no longer considered as a therapeutic agent for shigellosis, the fluoroquinolone ciprofloxacin is the current recommendation of the World Health Organization. Resistance to nalidixic acid is a marker of reduced susceptibility to older generation fluoroquinolones, such as ciprofloxacin. We aimed to assess the efficacy of gatifloxacin versus ciprofloxacin in the treatment of uncomplicated shigellosis in children. METHODOLOGY/PRINCIPAL FINDINGS: We conducted a randomized, open-label, controlled trial with two parallel arms at two hospitals in southern Vietnam. The study was designed as a superiority trial and children with dysentery meeting the inclusion criteria were invited to participate. Participants received either gatifloxacin (10 mg/kg/day) in a single daily dose for 3 days or ciprofloxacin (30 mg/kg/day) in two divided doses for 3 days. The primary outcome measure was treatment failure; secondary outcome measures were time to the cessation of individual symptoms. Four hundred and ninety four patients were randomized to receive either gatifloxacin (n=249) or ciprofloxacin (n=245), of which 107 had a positive Shigella stool culture. We could not demonstrate superiority of gatifloxacin and observed similar clinical failure rate in both groups (gatifloxacin; 12.0% and ciprofloxacin; 11.0%, p=0.72). The median (inter-quartile range) time from illness onset to cessation of all symptoms was 95 (66-126) hours for gatifloxacin recipients and 93 (68-120) hours for the ciprofloxacin recipients (Hazard Ratio [95%CI]=0.98 [0.82-1.17], p=0.83). CONCLUSIONS: We conclude that in Vietnam, where nalidixic acid resistant Shigellae are highly prevalent, ciprofloxacin and gatifloxacin are similarly effective for the treatment of acute shigellosis.